This proposal seeks to compare the mechanical characteristics of the systemic venous system of the dog and pig. The former animal has a physiologically active hepatic sphincter, the latter probably does not. In anesthetized animals, a right heart bypass preparation will provide separation of venous return into splanchnic and non-splanchnic vascular beds. Each channel will drain by gravity into an extracorporeal reservoir. Total venous return will then be pumped at constant flow into the pulmonary artery. Venous mechanical parameters will be determined from transient and steady-state shifts in blood volume which occur between animal and extracorporeal reservoir following step changes in venous pressure. A second objective of this proposal is to investigate the influence that changes in venous mechanical parameters have on transvascular movement of fluid between interstitial and vascular compartments and how this shift affects venous return. This will be done by comparing the segmental venous resistance which occurs from the effective midpoint of the exchange vessels to the right atrium (Rv) with the venous resistance which occurs from the effective midpoint of the capacitance vessels to the right atrium (Rel). To the extent that these venous resistances overlap their control must be linked. We will measure Rel by the mechanical method described above and Rv with a modification of the method Pappenheimer and Soto-Rivera which measures isovolumic capillary pressure. These experiments will allow us to identify how much of the information concerning the control of venous return in the dog can be transferred to other species and will contribute to our understanding of the linkage between intra- and extravascular fluid compartments. By means of these experiments we will also begin to integrate two large bodies of physiologic information, i.e. principles related to circulatory mechanics and those principles governing the hydraulic conductivity through capillary walls.